1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a gas concentration measuring apparatus designed to correct an error contained in an output of a gas concentration sensor.
2. Background Art
The air pollution caused by exhaust emissions of automotive internal combustion engines is giving rise to a serious problem at the present day. The exhaust emission control standard regulations have been made more rigorous recently. The burning control of gasoline or diesel engines or use of catalyst are, therefore, being studied to reduce pollutants contained in exhaust gasses. In U.S., OBD-II (On Board Diagnostic-II) requirements prescribe that automotive vehicles have a function of determining whether a catalytic converter is operating normally or not.
As one of systems meeting the OBD-II requirements, a two-O2 sensor monitoring system is proposed which monitors outputs of two O2 sensors mounted upstream and downstream of a catalytic converter, respectively, but it is not designed to detect pollutants directly and cannot determine whether pollutants in exhaust gasses have been reduced accurately or not.
If it becomes possible to measure the concentration of NOx in exhaust gasses for monitoring the burning control and the catalytic converter, the pollutants in the exhaust gasses can be reduced greatly. Specifically, the reduction in pollutants in exhaust emissions of the engine is achieved by controlling the quantity of fuel to be injected into the engine and the EGR rate based on the concentration of NOx contained in the exhaust gasses. Additionally, the determination of deterioration of the catalytic converter is achieved easily by installing a NOx sensor downstream of the catalytic converter.
NOx sensors capable of measuring the concentration of NOx accurately and techniques for mounting such NOx sensors in automotive vehicles are, therefore, being sought.
The effects of air-fuel ratio feedback control may be improved further by monitoring the concentration of O2 contained in the exhaust gasses as well as the concentration of NOx. Specifically, modern air-fuel ratio control for automotive vehicles is required to improve the accuracy of the control and perform lean burn engine control. For meeting these requirements, sensors capable of determining the air-fuel ratio of a mixture supplied to the engine over a wide range are preferable.
Keeping such sensors activated to maintain the accuracy of detection at a certain level requires a heater for keeping the temperature of the sensor at a constant value required for activation of the sensor. The heater may be built in the sensor. In this case, an insulator is disposed between the heater and a sensor element of the sensor. A supply of power to the heater is controlled cyclically by turning on and off a switch connecting a power supply and the heater. An increase in temperature of the sensor element by the heater, however, causes the resistance of the insulator of the sensor to drop, which will cause a leakage current to flow to the sensor element through the insulator during on-off control of the supply of power to the heater, resulting in addition of an error current to an output of the sensor element. Even if the resistance of the insulator is constant, an increase in voltage applied to the heater during the on-off control of the supply of power to the heater also results in an increase in leakage current.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas concentration measuring apparatus designed to correct an error contained in an output of a gas concentration sensor arising from control of supply of power to a heater built in the gas concentration sensor.
According to one aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a signal indicative of the concentration of a preselected component contained in gasses, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater; (b) a heater control circuit controlling a supply of power to the heater of the gas concentration sensor; and (c) an error correcting circuit correcting an error contained in the signal produced by the sensor element of the gas concentration sensor arising from a leakage current flowing into the sensor element through the insulator during control of the supply of power to the heater by the heater control circuit.
In the preferred mode of the invention, the heater control circuit determines a target voltage to be applied to the heater for keeping a temperature of the sensor element of the gas concentration sensor at a given value required for activation of the sensor element and controls the supply of power to the heater based on the target voltage.
The heater control circuit limits a change in voltage applied to the heater when the gas concentration sensor is in a activated state to below a given value.
The heater control circuit includes, a power supply, a switching element, a coil, and a capacitor. The switching element is turned on and off to apply a voltage of the power supply to the heater cyclically. The coil and the capacitor serve to smooth the voltage of the power supply.
The switching frequency of the switching element is 1 kHz or more.
A filter may be provided which cuts off low frequency components below 100 Hz from the signal produced by the sensor element of the gas concentration sensor.
The may cut off low frequency components below at least the switching frequency from the signal produced by the sensor element of the gas concentration sensor.
The sensor element may include a first cell which is responsive to application of voltage to discharge oxygen contained in the gasses and produces a current signal as a function of a concentration of the oxygen and a second cell which is responsive to application of voltage to produce a current signal as a function of a concentration of a given gas component contained in the gasses after the oxygen is discharged through the first cell.
The sensor element may alternatively has a cell which produces an electromotive force as the gas concentration signal.
According to the second aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a signal indicative of the concentration of a preselected component contained in gasses, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater; (b) a heater control circuit supplying power to the heater cyclically using a pulse-width modulated (PWM) signal; and (c) a filter passing frequency components of the signal produced by the sensor element within a given low frequency band. The frequency of the PWM signal is so determined as to allow the filter to compensate for an error which is contained in the signal inputted to the filter and which arises from the PWM signal.
In the preferred mode of the invention, the frequency of the PWM signal is ten or more times a cutoff frequency of the filter.
The PWM signal may be at least greater than a frequency of a change in signal outputted from the gas concentration sensor.
The cutoff frequency of the filter is less than or equal to 100 Hz.
The cutoff frequency of the filter may also be at least less than or equal to the frequency of the PWM signal.
A detecting circuit is provided which detects at least one of a voltage applied to the heater and a current flowing through the heater and a sample-and-hold circuit connected to an output of the detecting circuit.
The sensor element includes a first cell which is responsive to application of voltage to discharge oxygen contained in the gasses and produces a current signal as a function of a concentration of the oxygen and a second cell which is responsive to application of voltage to produce a current signal as a function of a concentration of a given gas component contained in the gasses after the oxygen is discharged through the first cell.
The sensor element may alternatively have a cell which produces an electromotive force as the gas concentration signal.
According to the third aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a signal indicative of the concentration of a preselected component contained in gasses, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater, the heater being connected at a first end to a power supply and at a second end to ground; (b) a heater control circuit controlling a supply of power from the power supply to the heater of the gas concentration sensor; and (c) a switch disposed between the first end of the heater and the power supply, the switch establishing communication between the heater and the power supply when the power is supplied to the heater through the heater control circuit while blocking the communication when the supply of power to the heater is cut.
In the preferred mode of the invention, a second switch may also be disposed between the second end of the heater and the ground. The second switch establishes communication between the heater and the ground when the power is supplied to the heater through the heater control circuit while blocking the communication when the supply of power to the heater is cut.
According to the fourth aspect of the invention, there is provided a gas measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a gas concentration signal as a function of concentration of a specified component of gasses to be measured, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater; (b) a heater control circuit controlling a supply of power to the heater of the gas concentration sensor in pulse-width modulation; and (c) a circuit detecting the gas concentration signal either for a power supply-on duration in which the power is supplied to the heater through the heater control circuit or for a power supply-off duration in which the supply of power to the heater is cut.
According to the fifth aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a gas concentration signal indicative of the concentration of a preselected component contained in gasses, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater; (b) a heater control circuit supplying power to the heater cyclically using a pulse-width modulated (PWM) signal; and (c) a correction circuit monitoring values of the gas concentration signal in a power supply-on duration for which the power is supplied to the heater and a power supply-off duration for which supply of the power to the heater is cut off, the correction circuit corrects the gas concentration signal using the monitored values.
In the preferred mode of the invention, the correction circuit averages the values of the gas concentration signal in the power supply-on duration and the power supply-off duration and corrects the gas concentration signal using an averaged value.
The sensor element includes a first cell which is responsive to application of voltage to discharge oxygen contained in the gasses and produces a current signal as a function of a concentration of the oxygen and a second cell which is responsive to application of voltage to produce a current signal as a function of a concentration of a given gas component contained in the gasses after the oxygen is discharged through the first cell.
The sensor element may alternatively have a cell which produces an electromotive force as the gas concentration signal.
According to the sixth aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a sensor element producing a gas concentration signal indicative of the concentration of a preselected component contained in gasses, a heater heating the sensor element, and an insulator disposed between the sensor element and the heater; (b) a heater control circuit supplying power to the heater cyclically using a pulse-width modulated (PWM) signal; and (c) a correction circuit estimating an error which is to be contained in the gas concentration signal and which arises from a leakage current flowing into the sensor element through the insulator caused by a change in resistance of the insulator produced during control of supply of the power to the heater by the heater control circuit using the PWM signal, the correction circuit removing the estimated error from the gas concentration signal.
In the preferred mode of the invention, the correction circuit corrects the gas concentration signal using a greater correction value as a voltage of a power supply for the heater increases.
The correction circuit corrects the gas concentration signal using a greater correction value as a temperature of the sensor element increases.
The sensor element includes a first cell which is responsive to application of voltage to discharge oxygen contained in the gasses and produces a current signal as a function of a concentration of the oxygen and a second cell which is responsive to application of voltage to produce a current signal as a function of a concentration of a given gas component contained in the gasses after the oxygen is discharged through the first cell.
The sensor element may alternatively have a cell which produces an electromotive force as the gas concentration signal.